Ink jet printers have printheads that operate a plurality of inkjets that eject liquid ink onto an image receiving member. The ink may be stored in reservoirs located within cartridges installed in the printer. Such ink may be aqueous ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the imaging member. In these solid ink printers, the solid ink may be in the form of pellets, ink sticks, granules or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device that melts the ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. In other inkjet printers, ink may be supplied in a gel form. The gel is also heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead.
A typical inkjet printer uses one or more printheads. Each printhead typically contains an array of individual nozzles for ejecting drops of ink across an open gap to an image receiving member to form an image. The image receiving member may be a continuous web of recording media, a series of media sheets, or the image receiving member may be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an orifice from an ink filled conduit in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, or voltage level, of the signals affects the amount of ink ejected in each drop. The firing signal is generated by a printhead controller in accordance with image data. An inkjet printer forms a printed image in accordance with the image data by printing a pattern of individual ink drops at particular locations on the image receiving member. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member in accordance with image data.
In order for the printed images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads must be registered with reference to the imaging surface and with the other printheads in the printer. Registration of printheads is a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the orientation of the printhead with reference to the imaging surface and with reference to the other printheads in the printer. Operating the printheads in a printer to eject ink in correspondence with image data presumes that the printheads are level with a width across the image receiving member and that all of the inkjet ejectors in the printhead are operational. The presumptions regarding the orientations of the printheads, however, cannot be assumed, but must be verified. Additionally, if the conditions for proper operation of the printheads cannot be verified, the analysis of the printed image should generate data that can be used either to adjust the printheads so they better conform to the presumed conditions for printing or to compensate for the deviations of the printheads from the presumed conditions.
Analysis of printed images is performed with reference to two directions. “Process direction” refers to the direction in which the image receiving member is moving as the imaging surface passes the printhead to receive the ejected ink and “cross-process direction” refers to the direction across the width of the image receiving member. In order to analyze a printed image, a test pattern needs to be generated so determinations can be made as to whether the inkjets operated to eject ink did, in fact, eject ink and whether the ejected ink landed where the ink would have landed if the printhead was oriented correctly with reference to the image receiving member and the other printheads in the printer. In some printing systems, an image of a printed image is generated by printing the printed image onto media or by transferring the printed image onto media, ejecting the media from the system, and then scanning the image with a flatbed scanner or other known offline imaging device. This method of generating a picture of the printed image suffers from the inability to analyze the printed image in situ and from the inaccuracies imposed by the external scanner. In some printers, a scanner is integrated into the printer and positioned at a location in the printer that enables an image of an ink image to be generated while the image is on media within the printer or while the ink image is on the rotating image member. These integrated scanners typically include one or more illumination sources and a plurality of optical detectors that receive radiation from the illumination source that has been reflected from the image receiving surface. The radiation from the illumination source is usually visible light, but the radiation may be at or beyond either end of the visible light spectrum. If light is reflected by a white surface, the reflected light has the same spectrum as the illuminating light. In some systems, ink on the imaging surface may absorb a portion of the incident light, which causes the reflected light to have a different spectrum. In addition, some inks may emit radiation in a different wavelength than the illuminating radiation, such as when an ink fluoresces in response to a stimulating radiation. Each optical sensor generates an electrical signal that corresponds to the intensity of the reflected light received by the detector. The electrical signals from the optical detectors may be converted to digital signals by analog/digital converters and provided as digital image data to an image processor.
One known optical imaging system is configured to detect, for example, the presence, intensity, and/or location of ink drops jetted onto the receiving member by the inkjets of the printhead assembly. The light source for the imaging system may be a single light emitting diode (LED) that is coupled to a light pipe that conveys light generated by the LED to one or more openings in the light pipe that direct light towards the image substrate. In one embodiment, three LEDs, one that generates green light, one that generates red light, and one that generates blue light are selectively activated so only one light shines at a time to direct light through the light pipe and be directed towards the image substrate. In another embodiment, the light source is a plurality of LEDs arranged in a linear array. The LEDs in this embodiment direct light towards the image substrate. The light source in this embodiment may include three linear arrays, one for each of the colors red, green, and blue. Alternatively, all of the LEDS may be arranged in a single linear array in a repeating sequence of the three colors. The LEDs of the light source may be coupled to a controller or some other control circuitry to activate the LEDs for image illumination.
The reflected light is measured by the light detector in the optical imaging system. The light sensor, in one embodiment, is a linear array of photosensitive devices, such as charge coupled devices (CODs). The photosensitive devices generate an electrical signal corresponding to the intensity or amount of light received by the photosensitive devices. The linear array that extends substantially across the width of the image receiving member. Alternatively, a shorter linear array may be configured to translate across the image substrate. For example, the linear array may be mounted to a movable carriage that translates across image receiving member.
Arranging the optical sensors on a linear array may produce inaccuracies in the image data generated by the optical imaging system. As shown in FIG. 8(a), image data are generated for a test pattern of dashes on an image member with a straight and properly positioned linear array. In FIG. 8(b), image data of a test pattern of dashes are generated by a linear array that is slanted across the face of the media being imaged. As a result, the image data indicate the printheads ejecting the ink drops onto the media are at different positions in the process direction, when, in fact, the printheads may be at the same position in the process direction. Another inaccuracy may arise if the linear array is not truly straight. If the member to which the optical detectors are mounted is bowed, then the image data again depicts the ink drops ejected by printheads in the area of a bow as not being aligned with other printheads in the process direction. An example of the image data generated by a bowed linear array is shown in FIG. 8(c). Another issue that may arise with the linear array of optical detectors is the orientation of the lenses that directed reflected light onto the optical detectors. If these lenses do not have their axes aligned with one another, waviness may appear in the image data as shown in FIG. 8(d). Detecting and compensating for these inaccuracies in image data produced by a linear array of optical detectors would be useful.